The biogeochemical fate of organic acids in a shallow aquifer contaminated with gasoline

A combined field and laboratory study of low-molecular-weight organic acids was undertaken to better understand the distribution of organic acids and the geochemical parameters that influence the prevalence of these compounds in ground water of a shallow Coastal Plain aquifer contaminated with gasoline, in Galloway Township, New Jersey. The geochemical evolution of the aquifer, near a former leaking gasoline tank, was studied over a two-year period (1989-1991). The degradation of aromatic hydrocarbons from gasoline occurred in situ by a number of oxidation-reduction reactions mediated by bacteria. The important reactions were aerobic degradation and, in the absence of oxygen, nitrate, sulfate, and Fe(III) reduction. These reactions resulted in steep chemical gradients in ground water and the formation of authigenic iron-sulfide minerals. Shifts in the biogeochemical reactions occurred over time and small-scale space in response to changes in hydrogeochemical conditions in the aquifer.; Organic acids were associated with hydrocarbon degradation in oxygen-depleted zones of the aquifer. The concentrations of low-molecular-weight organic acids, including aliphatic, aromatic and alicyclic monocarboxylic compounds, were highest in the spring of 1990, reaching approximately 4 mg/L in shallow ground water. Laboratory microcosm experiments, with sediment and water from the anoxic zone of the aquifer, demonstrated that the biogeochemical fate of specific organic acids observed in ground water varied with the structure of the acid and the availability of electron acceptors.; The concentrations of organic acids in two other shallow aquifers, contaminated with creosote waste and crude oil, were compared to the results at the Galloway site. The geochemical evolution of these aquifers was controlled by microbially mediated electron-transfer reactions that were driven by the availability of electron acceptors as well as degradable organic material. The organic acids identified at these sites were related to the geochemical conditions under which the degradation of dissolved organic compounds occurred. The increased understanding of the biogeochemical fate of organic acids in ground water is essential to predict the consequences of hydrocarbon degradation on a variety of water resources problems, including facilitated transport of metals and enhanced dissolution of aquifer minerals.